Methods and system for starting an engine of a hybrid vehicle

ABSTRACT

Systems and methods for cranking an engine of a hybrid vehicle that includes an electric machine to crank the engine and propel the vehicle are disclosed. In one example, engine cranking speed and engine cranking source are selected in response to vehicle operating conditions that may affect whether or not an electrical power source has sufficient energy to crank the engine.

FIELD

The present description relates to methods and a system for starting anengine of a hybrid vehicle during different operating conditions. Themethods may be particularly useful for hybrid vehicles that include adriveline disconnect clutch, electric motor, and an engine.

BACKGROUND AND SUMMARY

An engine of a hybrid vehicle may be started by rotating the engine witha high voltage electric machine and supplying spark and fuel to theengine. The electric machine may rotate the engine to a desired engineidle speed before the engine is supplied with spark and fuel.Alternatively, the electric machine may rotate the engine at a crankingspeed (e.g., 200 RPM) at which time spark and fuel may be supplied tothe engine. Some hybrid vehicles include a driveline disconnect clutchthat is positioned between the engine and the high voltage electricmachine. The driveline disconnect clutch allows the high voltageelectric machine to operate independently from the engine. Consequently,the vehicle has the capability of being propelled solely via the highvoltage electric machine. However, the driveline disconnect clutch maymake it more difficult to start the engine at cold ambient temperaturesbecause the driveline disconnect clutch requires a pump to supply itpressurized working fluid in order to close so that the engine may berotated. Consequently, the high voltage electric machine, engine, andthe working fluid pump may have to be rotated by the high voltageelectric machine at a time when the battery that provides power to thehigh voltage electric machine may exhibit reduced output power due atleast in part to a lower battery discharge limit.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a driveline method, comprising: adjusting an electricmachine to a first speed in response to a request to start an engine;adjusting the electric machine to a second speed after achieving thefirst speed in response to the request to start the engine; and startingan engine via closing a driveline disconnect clutch while or aftertransitioning the electric machine to the second speed in response tothe request to start the engine.

By rotating a driveline at a first speed before lowering driveline speedand cranking an engine at a second speed, it may be possible to providethe technical result of starting the engine at lower temperatures wherea battery supplying electrical power to the electric machine may havereduced discharge power limits (e.g., kW). For example, an electricmachine may be rotated at a first higher speed to provide working fluidpressure capable of closing a driveline disconnect clutch. After thedesired working fluid pressure is achieved, the electric machine speedmay be reduced to a speed where the engine may be cranked with lesstorque than if the engine were cranked at higher speeds. Consequently, adesired working fluid pressure to close a driveline disconnect clutchmay be achieved, and the engine may be cranked without the reducedenergy storage device discharge limits being exceeded.

The present description may provide several advantages. In particular,the approach may allow an engine to be started during conditions oflower energy storage device discharge limits. Further, the approach mayprovide alternative ways to start an engine of a hybrid vehicle. Furtherstill, the approach may reduce the possibility of vehicle componentdegradation.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 shows an example vehicle driveline configuration;

FIG. 3 shows an example engine starting sequence; and

FIG. 4 shows an example method for starting an engine of a hybridvehicle.

DETAILED DESCRIPTION

The present description is related to improving engine starting of ahybrid vehicle. The engine may be an engine as shown in FIG. 1 or adiesel engine. The engine may be included in a driveline of a hybridvehicle as is shown in FIG. 2. The engine may be started according tothe method of FIG. 4 as is shown in the sequence of FIG. 3. The methodof FIG. 4 provides different ways of starting an engine in response tovehicle operating conditions.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Flywheel 97 and ring gear 99 arecoupled to crankshaft 40. Starter 96 (e.g., low voltage (operated withless than 30 volts) electric machine) includes pinion shaft 98 andpinion gear 95. Pinion shaft 98 may selectively advance pinion gear 95to engage ring gear 99. Starter 96 may be directly mounted to the frontof the engine or the rear of the engine. In some examples, starter 96may selectively supply torque to crankshaft 40 via a belt or chain. Inone example, starter 96 is in a base state when not engaged to theengine crankshaft. Combustion chamber 30 is shown communicating withintake manifold 44 and exhaust manifold 48 via respective intake valve52 and exhaust valve 54. Each intake and exhaust valve may be operatedby an intake cam 51 and an exhaust cam 53. The position of intake cam 51may be determined by intake cam sensor 55. The position of exhaust cam53 may be determined by exhaust cam sensor 57.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Alternatively, fuel may be injected to an intake port, whichis known to those skilled in the art as port injection. Fuel injector 66delivers liquid fuel in proportion to the pulse width from controller12. Fuel is delivered to fuel injector 66 by a fuel system (not shown)including a fuel tank, fuel pump, and fuel rail (not shown).

In addition, intake manifold 44 is shown communicating with turbochargercompressor 162. Shaft 161 mechanically couples turbocharger turbine 164to turbocharger compressor 162. Optional electronic throttle 62 adjustsa position of throttle plate 64 to control air flow from air intake 42to compressor 162 and intake manifold 44. In one example, a highpressure, dual stage, fuel system may be used to generate higher fuelpressures. In some examples, throttle 62 and throttle plate 64 may bepositioned between intake valve 52 and intake manifold 44 such thatthrottle 62 is a port throttle.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106 (e.g., non-transitory memory), random access memory 108, keepalive memory 110, and a conventional data bus. Controller 12 is shownreceiving various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including: engine coolanttemperature (ECT) from temperature sensor 112 coupled to cooling sleeve114; a position sensor 134 coupled to an accelerator pedal 130 forsensing force applied by foot 132; a position sensor 154 coupled tobrake pedal 150 for sensing force applied by foot 152, a measurement ofengine manifold pressure (MAP) from pressure sensor 122 coupled tointake manifold 44; an engine position sensor from a Hall effect sensor118 sensing crankshaft 40 position; a measurement of air mass enteringthe engine from sensor 120; and a measurement of throttle position fromsensor 58. Barometric pressure may also be sensed (sensor not shown) forprocessing by controller 12. In a preferred aspect of the presentdescription, engine position sensor 118 produces a predetermined numberof equally spaced pulses every revolution of the crankshaft from whichengine speed (RPM) can be determined.

In some examples, the engine may be coupled to an electric motor/batterysystem in a hybrid vehicle as shown in FIG. 2. Further, in someexamples, other engine configurations may be employed, for example adiesel engine.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion. During the expansion stroke, the expanding gases pushpiston 36 back to BDC. Crankshaft 40 converts piston movement into arotational torque of the rotary shaft. Finally, during the exhauststroke, the exhaust valve 54 opens to release the combusted air-fuelmixture to exhaust manifold 48 and the piston returns to TDC. Note thatthe above is shown merely as an example, and that intake and exhaustvalve opening and/or closing timings may vary, such as to providepositive or negative valve overlap, late intake valve closing, orvarious other examples.

FIG. 2 is a block diagram of a vehicle 225 including a driveline 200.The driveline of FIG. 2 includes engine 10 shown in FIG. 1. Driveline200 may be powered by engine 10. Engine 10 may be started with an enginestarting system shown in FIG. 1 or via driveline integratedstarter/generator (DISG) 240. DISG 240 (e.g., high voltage (operatedwith greater than 30 volts) electrical machine) may also be referred toas an electric machine, motor, and/or generator. Further, torque ofengine 10 may be adjusted via torque actuator 204, such as a fuelinjector, throttle, etc.

An engine output torque may be transmitted to an input side of drivelinedisconnect clutch 236 through dual mass flywheel 215. Disconnect clutch236 may be electrically or hydraulically actuated. If disconnect clutch236 is hydraulically actuated, pump 213 supplies working fluid (e.g.,oil) to driveline disconnect clutch 236. Pump 213 may be incorporatedinto torque converter 206 or transmission 208, and pump 213 rotates tosupply pressurized working fluid to driveline disconnect clutch 236 andclutches 210-211. Pump 213 is mechanically driven and it rotates topressurize working fluid when shaft 241 rotates. Pressure at an outletof pump 213 may be determined via pressure sensor 214. The downstreamside of disconnect clutch 236 is shown mechanically coupled to DISGinput shaft 237.

DISG 240 may be operated to provide torque to driveline 200 or toconvert driveline torque into electrical energy to be stored in electricenergy storage device 275. DISG 240 has a higher output torque capacitythan starter 96 shown in FIG. 1. Further, DISG 240 directly drivesdriveline 200 or is directly driven by driveline 200. There are nobelts, gears, or chains to couple DISG 240 to driveline 200. Rather,DISG 240 rotates at the same rate as driveline 200. Electrical energystorage device 275 (e.g., high voltage battery or power source) may be abattery, capacitor, or inductor. The downstream side of DISG 240 ismechanically coupled to the impeller 285 of torque converter 206 viashaft 241. The upstream side of the DISG 240 is mechanically coupled tothe disconnect clutch 236.

Torque converter 206 includes a turbine 286 to output torque to inputshaft 270. Input shaft 270 mechanically couples torque converter 206 toautomatic transmission 208. Torque converter 206 also includes a torqueconverter bypass lock-up clutch 212 (TCC). Torque is directlytransferred from impeller 285 to turbine 286 when TCC is locked. TCC iselectrically operated by controller 12. Alternatively, TCC may behydraulically locked. In one example, the torque converter may bereferred to as a component of the transmission.

When torque converter lock-up clutch 212 is fully disengaged, torqueconverter 206 transmits engine torque to automatic transmission 208 viafluid transfer between the torque converter turbine 286 and torqueconverter impeller 285, thereby enabling torque multiplication. Incontrast, when torque converter lock-up clutch 212 is fully engaged, theengine output torque is directly transferred via the torque converterclutch to an input shaft (not shown) of transmission 208. Alternatively,the torque converter lock-up clutch 212 may be partially engaged,thereby enabling the amount of torque directly relayed to thetransmission to be adjusted. The controller 12 may be configured toadjust the amount of torque transmitted by torque converter 212 byadjusting the torque converter lock-up clutch in response to variousengine operating conditions, or based on a driver-based engine operationrequest.

Automatic transmission 208 includes gear clutches (e.g., gears 1-6) 211and forward clutch 210. The gear clutches 211 and the forward clutch 210may be selectively engaged to propel a vehicle. Torque output from theautomatic transmission 208 may in turn be relayed to wheels 216 topropel the vehicle via output shaft 260. Specifically, automatictransmission 208 may transfer an input driving torque at the input shaft270 responsive to a vehicle traveling condition before transmitting anoutput driving torque to the wheels 216. Further, a frictional force maybe applied to wheels 216 by engaging wheel brakes 218.

In one example, wheel brakes 218 may be engaged in response to thedriver pressing his foot on a brake pedal (not shown). In otherexamples, controller 12 or a controller linked to controller 12 mayapply engage wheel brakes. In the same way, a frictional force may bereduced to wheels 216 by disengaging wheel brakes 218 in response to thedriver releasing his foot from a brake pedal. Further, vehicle brakesmay apply a frictional force to wheels 216 via controller 12 as part ofan automated engine stopping procedure.

Controller 12 may be configured to receive inputs from engine 10, asshown in more detail in FIG. 1, and accordingly control a torque outputof the engine and/or operation of the torque converter, transmission,DISG, clutches, and/or brakes. As one example, an engine torque outputmay be controlled by adjusting a combination of spark timing, fuel pulsewidth, fuel pulse timing, and/or air charge, by controlling throttleopening and/or valve timing, valve lift and boost for turbo- orsuper-charged engines. In the case of a diesel engine, controller 12 maycontrol the engine torque output by controlling a combination of fuelpulse width, fuel pulse timing, and air charge. In all cases, enginecontrol may be performed on a cylinder-by-cylinder basis to control theengine torque output. Controller 12 may also control torque output andelectrical energy production from DISG by adjusting current flowing toand from field and/or armature windings of DISG as is known in the art.

When idle-stop conditions are satisfied, controller 12 may initiateengine shutdown by shutting off fuel and spark to the engine. However,the engine may continue to rotate in some examples. Further, to maintainan amount of torsion in the transmission, the controller 12 may groundrotating elements of transmission 208 to a case 259 of the transmissionand thereby to the frame of the vehicle. When engine restart conditionsare satisfied, and/or a vehicle operator wants to launch the vehicle,controller 12 may reactivate engine 10 by craning engine 10 and resumingcylinder combustion.

Thus, the system of FIGS. 1 and 2 provides for a driveline system,comprising: an engine; a low voltage electrical machine mechanicallycoupled to the engine; a high voltage electric machine; a drivelinedisconnect clutch positioned between the engine and the high voltageelectric machine; and a controller including executable instructionsstored in non-transitory memory for cranking the engine via the highvoltage electric machine or the low voltage electric machine in responseto a temperature, and further instructions for adjusting a crankingspeed of the engine in response to the temperature only when the highvoltage electric machine cranks the engine. The driveline systemincludes where the cranking speed of the engine is an engine speed wherespark and fuel are first supplied to the engine after an engine stop.The driveline system includes where the high voltage electric machinecranks the engine at a second speed after achieving a first speed inresponse to an engine start request when the temperature is less than athreshold temperature, the first speed greater than the second speed.The driveline system includes where the high voltage electric machinecranks the engine without engine speed being reduced after an enginestop when the temperature is greater than the threshold temperature. Thedriveline system further comprises additional instructions for reducingthe cranking speed of the engine in response to battery power limits.

Referring now to FIG. 3, an example engine starting sequence for ahybrid vehicle during cold operating conditions is shown. The sequenceof FIG. 3 may be provided by the system of FIGS. 1 and 2 executing themethod of FIG. 4.

The first plot from the top of FIG. 3 is a plot of high voltage electricmachine speed (DISG) and engine speed versus time. The high voltageelectric machine speed and engine speed increase in the direction of theY axis arrow. High voltage electric machine speed is indicated by thesolid trace 302. Engine speed is represented by dotted line 304. The Xaxis represents time and time increases from the left side of FIG. 3 tothe right side of FIG. 3.

The second plot from the top of FIG. 3 is a plot of driveline disconnectclutch pressure versus time. Driveline disconnect clutch closing forcemay be substituted for driveline disconnect clutch pressure. The Y axisrepresents driveline disconnect clutch pressure and pressure increasesin the direction of the Y axis arrow. The driveline disconnect clutch isclosed when the disconnect clutch pressure trace is near the Y axisarrow and open when the driveline disconnect pressure trace is near theX axis. The X axis represents time and time increases from the left sideof FIG. 3 to the right side of FIG. 3.

The third plot from the top of FIG. 3 is a plot of high voltage energysource (e.g., battery) discharge power versus time. The Y axisrepresents high voltage energy source discharge power and dischargepower increases in the direction of the Y axis arrow. Discharge power isnegative below the X axis. The X axis represents time and time increasesfrom the left side of FIG. 3 to the right side of FIG. 3.

The fourth plot from the top of FIG. 3 is a plot of engine torque andhigh voltage electric machine torque versus time. The Y axis representstorque and torque increases in the direction of the Y axis arrow. Highvoltage machine torque is represented by solid line 306 and enginetorque is represented by dotted line 308. The X axis represents time andtime increases from the left side of FIG. 3 to the right side of FIG. 3.

The fifth plot from the top of FIG. 3 is a plot of an engine startrequest versus time. The engine start request is asserted when the traceis at a higher level near the Y axis arrow. The engine start request isnot asserted when the trace is at a lower level near the X axis. The Xaxis represents time and time increases from the left side of FIG. 3 tothe right side of FIG. 3.

At time T0, the DISG speed and the engine speed are zero indicating thatthe vehicle is stopped. The driveline disconnect clutch is in an openstate allowing the DISG to rotate independently and/or without rotatingengine. The high voltage battery discharge power is also zero indicatingthat the high voltage battery is not being discharged. The DISG torqueand engine torque are also zero. The engine start request is notasserted.

At time T1, the engine start request is asserted in response to a driveror controller request. The engine and DISG temperature are at a lowlevel (not shown) and high voltage battery discharge power may also below. However, DISG torque and current may be elevated during suchconditions when rotating the engine due to increased engine friction,oil viscosity, and battery operating characteristics. The DISG speed isincreased to a first speed and the disconnect clutch remains open.Engine speed remains at zero. The DISG rotates pump 213 which suppliespressurized working fluid to the driveline disconnect clutch 236. Thehigh voltage battery discharge power increases as the DISG speedincreases and the DISG torque increases. The engine torque remains atzero since the driveline disconnect clutch is open.

At time T2, the DISG speed is decreased to a second speed and thedriveline disconnect clutch pressure begins to increase to close thedriveline disconnect clutch so that torque may be transferred from theDISG to the stopped engine. The DISG speed may be decreased to thesecond speed after a predetermined amount of time since the DISG reachedthe first speed has elapsed. Alternatively, the DISG speed may bedecreased to the second speed in response to working fluid pressure atan outlet of pump 213 reaching a threshold pressure. The thresholdpressure may be a pressure sufficient to close the driveline disconnectclutch. Reducing the DISG speed to the second speed may reduce theamount of torque the DISG needs to rotate the engine as compared to ifthe DISG were to rotate the engine at the first speed. Further, the DISGmay have additional torque capacity at the second speed as compared tothe first speed depending on the first and second speeds and the DISGoutput torque characteristics. The high voltage battery discharge powerincreases as DISG torque is increased to maintain DISG speed at thesecond speed as the driveline disconnect clutch is closed. The enginetorque increases in a negative direction as the engine begins to rotatein response to the driveline disconnect clutch closing. The engine startrequest remains asserted.

At time T3, the driveline disconnect clutch is fully closed and the DISGspeed reaches the second speed. The high voltage battery discharge powerlevels off to a constant value as the engine rotates at the secondspeed. The DISG torque also levels off at a constant torque that ittakes to crank the engine at the second speed and rotate the torqueconverter impeller. Spark and fuel (not shown) are supplied to theengine so that the engine may be started. The engine start requestremains asserted.

At time T4, the engine begins to combust air-fuel mixtures and theengine begins to accelerate the DISG. The DISG torque is reduced and itmoves toward a negative torque. The high voltage battery discharge alsobegins to be reduced as the DISG output torque is reduced in response tothe engine accelerating. The driveline disconnect clutch remains lockedand engine torque increases in a positive direction. The engine startrequest remains asserted.

In this way, it may be possible to first rotate the driveline at a speedwhere a desired working fluid pressure to close the driveline disconnectclutch is provided by a pump that is mechanically driven by thedriveline. Further, the driveline speed may then be reduced to a speedwhere engine friction may be reduced and/or DISG output torque may beincreased so that the DISG may have sufficient torque to rotate theengine at lower ambient temperatures. Thus, working fluid pressure maybe increased and engine friction may be held to a lower value to improvethe possibility of engine starting at low ambient temperatures.

Referring now to FIG. 4, a method for starting an engine of a hybridvehicle is shown. The method of FIG. 4 may provide the operatingsequence shown in FIG. 3. Additionally, the method of FIG. 4 may beincluded in the system of FIGS. 1 and 2 as executable instructionsstored in non-transitory memory.

At 402, method 400 judges if there is an engine start request. An enginestart request may be originated by a driver or a controller. A drivermay initiate an engine start request by turning a key or operating apushbutton. A controller may initiate an engine start request viachanging state of a variable in memory or a state of an output. Ifmethod 400 judges that there is an engine start request, the answer isyes and method 400 proceeds to 404. Otherwise, method 400 proceeds toexit.

At 404, method 400 judges if a temperature is less than a thresholdtemperature. In one example, the temperature is an engine temperature.In another example, the temperature is a DISG temperature. In stillanother example, the temperature is an energy storage devicetemperature. In still other examples, method 400 may select a lowesttemperature from a group of devices not limited to the DISG, battery,engine, ambient air, and oil as a basis for comparing to the thresholdtemperature. If method 400 judges that the temperature is less than thethreshold temperature, the answer is yes and method 400 proceeds to 404.Otherwise, the answer is no and method 400 proceeds to 412.

At 406, method 400 cranks the engine via a low voltage electricalmachine or a starter motor 96. The starter motor is supplied power by alow voltage battery (e.g., less than 30 volts). The low voltageelectrical machine may rotate the engine at a speed of less than 300RPM. The low voltage starter may be supplied electrical power from asource different than the high voltage energy storage device. Therefore,the low voltage starter may be able to generate more torque than theDISG during some operating conditions such as when reduced batterydischarge limits are in effect. Method 400 proceeds to 408 afterbeginning to crank the engine.

At 408, method 400 supplies spark and fuel to start the engine as theengine is cranked (e.g., rotated) by the low voltage starter. The lowvoltage starter may rotate the engine at a speed that is different froma speed that the DISG rotates the engine during engine starting. Method400 proceeds to exit after the engine is started.

At 410, method 400 judges whether or not a battery discharge power limitis less than a threshold. The battery discharge power limit may varywith battery temperature, battery state of charge (SOC), and otherconditions. If method 400 judges that the high voltage battery dischargepower limit is less than a threshold, the answer is yes and method 400proceeds to 420. Otherwise, the answer is no and method 400 proceeds to412.

At 412, method 400 adjusts DISG to a first speed. The first speed may bea speed at which pump 213 provides sufficient pressure to close thedriveline disconnect clutch (e.g., 300 RPM) in a predetermined amount oftime. Method 400 proceeds to 414 after the DISG is adjusted to the firstspeed.

At 414, method 400 starts to close the driveline disconnect clutch. Thedriveline disconnect clutch may be closed by allowing working fluid toreach the driveline disconnect clutch from pump 213. Method 400 proceedsto 416 after the driveline disconnect clutch begins to close.

At 416, method 400 starts the engine via rotating the engine via theDISG and by supplying spark and fuel to the engine. Spark and fuel maybe supplied to the engine as soon as engine position is determined.Method 400 proceeds to exit after the engine is started by rotating theengine at a single cranking speed.

At 420, method 400 adjusts DISG speed to a second speed. In one example,the DISG speed may be adjusted to a speed at which pump 213 providessufficient pressure to close the driveline disconnect clutch (e.g., 300RPM) in a predetermined amount of time. The second speed may bedifferent than the first speed described at 412. Further, the secondspeed may be adjusted or calibrated to different levels based on vehicleoperating conditions. Method 400 proceeds to 422 after the DISG speed isadjusted to the first speed.

At 422, method 400 judges whether or not select conditions have beenmet. In one example, the select condition may be that a threshold timefrom a time the DISG reaches the first speed. In another example, theselect condition may be that an outlet pressure of pump 213 has reacheda threshold pressure. If method 400 judges that one or more selectedconditions have been met, the answer is yes and method 400 proceeds to424. Otherwise, the answer is no and method 400 returns to 422.

At 424, method 400 reduces the DISG to a third speed. In one example,the third speed is a speed where engine friction is less than athreshold value and where engine speed is greater than a thresholdspeed. The third speed is also less than the second speed, and the thirdspeed may be calibrated or adjusted to different speeds based onoperating conditions. Method 400 proceeds to 426 after DISG speed beginsto be adjusted to the third speed.

At 426, method 400 begins to close the driveline disconnect clutch. Thedriveline disconnect clutch may be closed via supplying a working fluidto the driveline disconnect clutch. Method 400 proceeds to 428 after thedriveline disconnect clutch begins to close.

At 428, method 400 starts the engine via the DISG when DISG speed is atthe third speed. The engine is started by supplying spark and fuel tothe engine. The driveline disconnect clutch transfers torque from theDISG to the engine when the engine is being started. The engine speedmay be ramped to idle speed after engine run-up from cranking speed.Method 400 proceeds to exit after the engine is started.

In this way, an engine of a hybrid vehicle may be started via differentelectric machines in response to vehicle operating conditions. Further,speeds of the electric machine may be varied between two or moresubstantially constant speeds during the engine starting process. Byoperating the driveline at different speeds during engine starting, itmay be possible to start the engine even when battery discharging limitsare reduced.

Thus, the method of FIG. 4 provides for a driveline method, comprising:adjusting an electric machine to a first speed in response to a requestto start an engine; adjusting the electric machine to a second speedafter achieving the first speed in response to the request to start theengine; and starting an engine via closing a driveline disconnect clutchwhile or after transitioning the electric machine to the second speed inresponse to the request to start the engine. The method includes wherethe engine is at rest when the electric machine is adjusted to the firstspeed. The method includes where the electric machine is adjusted to thesecond speed in further response to an outlet pressure of a pump. Themethod includes where the electric machine is adjusted to the secondspeed in further response to a predetermined amount of time since thefirst speed was achieved.

In some examples, the method includes where the first speed is greaterthan the second speed. The method also includes where closing thedriveline disconnect clutch while transitioning the electric machine tothe second speed includes closing the driveline disconnect clutch afterthe electric machine achieves the second speed. The method furthercomprises adjusting engine speed to a desired idle speed after theengine is started.

The method of FIG. 4 also provides for a driveline method, comprising:in response to a first engine start request, adjusting a speed of anelectric machine to a first speed and closing a driveline disconnectclutch; and in response to a second engine start request, adjusting thespeed of the electric machine to a second speed, adjusting the speed ofthe electric machine to a third speed after achieving the second speed,and closing the driveline disconnect clutch during transitioning fromthe second speed to the third speed. The method further comprisesadjusting the speed of an electric machine to the first speed andclosing the driveline disconnect clutch in response to a temperature ofa device during the first engine start request.

In some examples, the method further comprises adjusting the speed ofthe electric machine to the second speed, adjusting the speed of theelectric machine to the third speed after achieving the second speed,and closing the driveline disconnect clutch during transitioning fromthe second speed to the third speed in response to the temperature ofthe device during the second engine start request. The method alsoincludes where the device is an engine. The method includes where thedevice is an energy storage device or the electric machine. The methodfurther comprises starting an engine after beginning to close thedriveline disconnect clutch in response to the first and second enginestart requests. The method includes where the second speed is greaterthan the third speed. The method includes where the electric machine isa high voltage electrical machine, and further comprising starting theengine via a low voltage electric machine in response to a third enginestart request at an ambient temperature that is less than ambienttemperature during the first and second engine start requests.

As will be appreciated by one of ordinary skill in the art, the methodsdescribed in FIG. 4 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described actions,operations, methods, and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the engine control system.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

1. A driveline method, comprising: adjusting an electric machine to afirst speed in response to a request to start an engine; adjusting theelectric machine to a second speed after achieving the first speed inresponse to the request to start the engine; and starting an engine viaclosing a driveline disconnect clutch while or after transitioning theelectric machine to the second speed in response to the request to startthe engine.
 2. The method of claim 1, where the engine is at rest whenthe electric machine is adjusted to the first speed.
 3. The method ofclaim 1, where the electric machine is adjusted to the second speed infurther response to an outlet pressure of a pump.
 4. The method of claim1, where the electric machine is adjusted to the second speed in furtherresponse to a predetermined amount of time since the first speed wasachieved.
 5. The method of claim 1, where the first speed is greaterthan the second speed.
 6. The method of claim 1, where closing thedriveline disconnect clutch while transitioning the electric machine tothe second speed includes closing the driveline disconnect clutch afterthe electric machine achieves the second speed.
 7. The method of claim1, further comprising adjusting engine speed to a desired idle speedafter the engine is started. 8-15. (canceled)
 16. A driveline system,comprising: an engine; a low voltage electrical machine mechanicallycoupled to the engine; a high voltage electric machine; a drivelinedisconnect clutch positioned between the engine and the high voltageelectric machine; and a controller including executable instructionsstored in non-transitory memory for cranking the engine via the highvoltage electric machine or the low voltage electric machine in responseto a temperature, and further instructions for adjusting a crankingspeed of the engine in response to the temperature only when the highvoltage electric machine cranks the engine.
 17. The driveline system ofclaim 16, where the cranking speed of the engine is an engine speedwhere spark and fuel are first supplied to the engine after an enginestop.
 18. The driveline system of claim 16, where the high voltageelectric machine cranks the engine at a second speed after achieving afirst speed in response to an engine start request when the temperatureis less than a threshold temperature, the first speed greater than thesecond speed.
 19. The driveline system of claim 18, where the highvoltage electric machine cranks the engine without engine speed beingreduced after an engine stop when the temperature is greater than thethreshold temperature.
 20. The driveline system of claim 15, furthercomprising additional instructions for reducing the cranking speed ofthe engine in response to battery power limits.